Atara Biotherapeutics (ATRA) 10-Q 8 Aug 16 2016 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2016	Jul. 31, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q2
Trading Symbol	ATRA
Entity Registrant Name	Atara Biotherapeutics, Inc.
Entity Central Index Key	1,604,464
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		28,811,019

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 26,356	$ 23,746
Short-term investments	268,201	296,736
Restricted cash	194	194
Prepaid expenses and other current assets	4,132	3,921
Total current assets	298,883	324,597
Property and equipment, net	1,830	270
Other assets	101	108
Total assets	300,814	324,975
Current liabilities:
Accounts payable	3,235	1,445
Accrued compensation	2,135	2,624
Accrued research and development expenses	3,799	5,112
Other accrued liabilities	837	528
Total current liabilities	10,006	9,709
Long-term liabilities	563	166
Total liabilities	10,569	9,875
Commitments and contingencies (Note 6)
Stockholders’ equity:
Common stock—$0.0001 par value, 500,000,000 shares authorized as of June 30, 2016 and December 31, 2015; 28,705,875 and 28,458,807 shares issued and outstanding as of June 30, 2016 and December 31, 2015, respectively	3	3
Additional paid-in capital	423,600	413,725
Accumulated other comprehensive income (loss)	193	(518)
Accumulated deficit	(133,551)	(98,110)
Total stockholders’ equity	290,245	315,100
Total liabilities and stockholders’ equity	$ 300,814	$ 324,975

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	28,705,875	28,458,807
Common stock, shares outstanding	28,705,875	28,458,807

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Operating expenses:
Research and development	$ 12,991	$ 11,507	$ 24,238	$ 17,274
General and administrative	6,494	3,601	12,308	7,145
Total operating expenses	19,485	15,108	36,546	24,419
Loss from operations	(19,485)	(15,108)	(36,546)	(24,419)
Interest and other income, net	605	163	1,108	316
Loss before provision for income taxes	(18,880)	(14,945)	(35,438)	(24,103)
Provision for income taxes			3	2
Net loss	(18,880)	(14,945)	(35,441)	(24,105)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale securities	142	(48)	711	34
Comprehensive loss	$ (18,738)	$ (14,993)	$ (34,730)	$ (24,071)
Net loss per common share:
Basic and diluted net loss per common share	$ (0.66)	$ (0.62)	$ (1.24)	$ (1.04)
Weighted-average common shares outstanding used to calculate basic and diluted net loss per common share	28,665	24,224	28,603	23,079

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Operating activities
Net loss	$ (35,441)	$ (24,105)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,860	5,053
Amortization of investment premiums and discounts	2,294	880
Depreciation expense	79	11
Loss on foreign exchange	4
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(816)	(3,314)
Other assets	7	(24)
Accounts payable	1,790	1,162
Accrued compensation	(489)	(301)
Accrued research and development expenses	(1,313)	4,500
Other accrued liabilities	235	3,247
Long-term liabilities	511	27
Net cash used in operating activities	(23,279)	(12,864)
Investing activities
Purchases of short-term investments	(186,006)	(111,325)
Sales of short-term investments	95,242	29,589
Maturities of short-term investments	118,320	33,920
Purchases of property and equipment	(1,639)	(5)
Net cash provided by (used in) investing activities	25,917	(47,821)
Financing activities
Proceeds from sale of common stock, net of offering costs		69,445
Taxes paid related to net share settlement of restricted stock units	(53)	(4,468)
Proceeds from exercise of stock options	29
Net cash provided by (used in) financing activities	(24)	64,977
Effect of exchange rates on cash	(4)
Increase in cash and cash equivalents	2,610	4,292
Cash and cash equivalents at beginning of period	23,746	21,897
Cash and cash equivalents at end of period	26,356	26,189
Non-cash investing and financing activities
Issuance of common stock upon vesting of stock awards	40	40
Change in long-term liabilities related to non-vested stock awards	(40)	(40)
Proceeds receivable from option exercises		32
Offering costs in anticipation of public filing included in other accrued liabilities and accounts payable		312
Property and equipment purchases included in liabilities	503
Supplemental cash flow disclosure
Cash paid for taxes	$ 3	$ 2

Description of Business

Description of Business	6 Months Ended
	Jun. 30, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of Business	1. Description of Business Atara Biotherapeutics, Inc. (“Atara”, “we”, “our” or “the Company”) was incorporated in August 2012 in Delaware. Atara is a clinical-stage biopharmaceutical company focused on developing meaningful therapies for patients with . Our T-cell programs were acquired through licensing arrangements with Memorial Sloan Kettering Cancer Center (“MSK”). Our molecularly targeted biologics programs were acquired through licensing arrangements with Amgen Inc. (“Amgen”). See Note 5 for further information.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (U.S. GAAP) and following the requirements of the Securities and Exchange Commission (SEC) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015, and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s consolidated financial information. The results of operations for the six month period ended June 30, 2016 are not necessarily indicative of the results to be expected for the full year or any other future period. The balance sheet as of December 31, 2015 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete consolidated financial statements. Significant Risks and Uncertainties We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. As of June 30, 2016, we had an accumulated deficit of $133.6 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. Management expects that our cash, cash equivalents and short-term investments as of June 30, 2016 will be sufficient to fund our planned operations through 2018. Concentration of Credit Risk and Other Uncertainties We place cash and cash equivalents in the custody of financial institutions that management believes are of high credit quality, the amount of which at times, may be in excess of the amount insured by the Federal Deposit Insurance Corporation. We also have short-term investments in money market funds, U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities, which can be subject to certain credit risk. However, we mitigate the risks by investing in high-grade instruments, limiting our exposure to any one issuer, and monitoring the ongoing creditworthiness of the financial institutions and issuers. We are subject to certain risks and uncertainties and believe that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: our ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, our product candidates, if approved; performance of third-party clinical research organizations and manufacturers upon which we rely; development of sales channels; protection of our intellectual property; litigation or claims against us based on intellectual property, patent, product, regulatory or other factors; and our ability to attract and retain employees necessary to support our growth. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical trial accruals and stock-based compensation expense. Actual results could differ materially from those estimates. Recent Accounting Pronouncements In January 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Loss (Topic 326): Measurement of Credit Losses on Financial Instruments

Net Loss per Common Share

Net Loss per Common Share	6 Months Ended
	Jun. 30, 2016
Earnings Per Share [Abstract]
Net Loss per Common Share	3. Basic net loss per common share is calculated by dividing net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration of common share equivalents. Diluted net loss per common share is computed by dividing net loss by the weighted-average number of shares of common stock and common share equivalents outstanding for the period. Common share equivalents are only included in the calculation of diluted net loss per common share when their effect is dilutive. Potential dilutive securities, which include unvested restricted stock awards (“RSAs”), unvested restricted stock units (“RSUs”), vested and unvested options to purchase common stock (“options”) and shares to be issued under our employee stock purchase plan (“ESPP”) have been excluded from the computation of diluted net loss per share as the effect is antidilutive. Therefore, the denominator used to calculate both basic and diluted net loss per common share is the same in all periods presented. The following table represents the potential common shares issuable pursuant to outstanding securities as of the related period end dates that were excluded from the computation of diluted net loss per common share as their inclusion would have an antidilutive effect: As of June 30, 2016 2015 Unvested RSAs 90,144 416,207 Unvested RSUs 922,569 564,821 Vested and unvested options 3,424,099 470,094 ESPP share purchase rights 4,048 —

Financial Instruments

Financial Instruments	6 Months Ended
	Jun. 30, 2016
Financial Instruments Disclosure [Abstract]
Financial Instruments	4. Our financial assets are measured at fair value on a recurring basis using the following hierarchy to prioritize valuation inputs, Level 1: Quoted prices in active markets for identical assets or liabilities that we have the ability to access Level 2: Observable market based inputs or unobservable inputs that are corroborated by market data such as quoted prices, interest rates and yield curves Level 3: Inputs that are unobservable data points that are not corroborated by market data We review the fair value hierarchy classification on a quarterly basis. Changes in the ability to observe valuation inputs may result in a reclassification of levels of certain securities within the fair value hierarchy. We recognize transfers into and out of levels within the fair value hierarchy in the period in which the actual event or change in circumstances that caused the transfer occurs. There have been no transfers between Level 1, Level 2, and Level 3 in any periods presented. The following tables summarize the estimated fair value and related valuation input hierarchy of our financial assets measured on a recurring basis, which were comprised solely of available-for-sale securities as of each period end: Total Total Total Total Amortized Unrealized Unrealized Estimated As of June 30, 2016: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 21,377 $ — $ — $ 21,377 U.S. Treasury obligations Level 2 81,460 114 (3 ) 81,571 Government agency obligations Level 2 29,716 25 (6 ) 29,735 Corporate debt obligations Level 2 154,173 109 (47 ) 154,235 Asset-backed securities Level 2 7,595 4 (2 ) 7,597 Total available-for-sale securities 294,321 252 (58 ) 294,515 Less amounts classified as cash equivalents (26,314 ) — — (26,314 ) Amounts classified as short-term securities $ 268,007 $ 252 $ (58 ) $ 268,201 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2015: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 16,364 $ — $ — $ 16,364 U.S. Treasury obligations Level 2 599 — (1 ) 598 Government agency obligations Level 2 36,480 1 (88 ) 36,393 Corporate debt obligations Level 2 203,767 8 (339 ) 203,436 Commercial paper Level 2 999 — — 999 Asset-backed securities Level 2 61,304 2 (102 ) 61,204 Total available-for-sale securities 319,513 11 (530 ) 318,994 Less amounts classified as cash equivalents (22,259 ) — 1 (22,258 ) Amounts classified as short-term securities $ 297,254 $ 11 $ (529 ) $ 296,736 The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of June 30, 2016 As of December 31, 2015 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 213,377 $ 213,431 $ 211,311 $ 211,059 Maturing in one to five years 80,944 81,084 108,202 107,935 Total available-for-sale securities $ 294,321 $ 294,515 $ 319,513 $ 318,994 As of June 30, 2016, certain available-for-sale securities had been in a continuous unrealized loss position, each for less than twelve months. As of this date, no significant facts or circumstances were present to indicate a deterioration in the creditworthiness of the respective issuers, and the Company had no requirement or intention to sell these securities before maturity or recovery of their amortized cost basis. During , we did not recognize any other-than-temporary impairment loss.

License and Collaboration Agree

License and Collaboration Agreements	6 Months Ended
	Jun. 30, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License and Collaboration Agreements	5. License and Collaboration Agreements MSK Agreements – In September 2014, we entered into an exclusive option agreement with MSK under which we had the right to acquire the exclusive worldwide license rights to three clinical stage T-cell therapies from MSK. In exchange for the option, we paid $1.25 million in cash and issued 59,761 shares of our common stock to MSK, which at the time of issuance had an estimated fair value of $0.75 million. The total of $2.0 million was recorded as research and development expense in our statements of operations and comprehensive loss. In June 2015, we exercised our option and entered into an exclusive license agreement with MSK. In connection with the execution of the license agreement, we paid was recorded as research and development expense in our statement of operations and comprehensive loss. We are required to make additional payments of up to $33.0 million to MSK based on achievement of specified regulatory and sales-related milestones, mid-single-digit percentage tiered royalty payments based on In addition, under certain circumstances, we are required to make certain minimum annual royalty payments to MSK, which are creditable against earned royalties owed for the same annual period. We are also required to pay a low double-digit percentage of any consideration we receive for sublicensing the licensed rights. Amgen License - In September 2012, we entered into three license agreements with Amgen. In accordance with terms of the agreements with Amgen, we use commercially reasonable efforts to prepare, file, prosecute, defend and maintain the patents covered by the license agreements. During the three months ended June 30, 2016 and 2015, we incurred expenses of $0.2 million and $0.3 million, respectively, related to these activities. During the six months ended June 30, 2016 and 2015, we incurred expenses of $0.4 million and $0.7 million, respectively, related to these activities. In December 2015, we announced that we would be suspending further development of PINTA 745 and in June 2016, we returned the rights related to this and the ATA 842 program to Amgen. Under the remaining license agreements, potential payments of up to $58.0 million are due to Amgen upon the achievement of development and regulatory approval milestones and payments of up to $104.0 million are due upon the achievement of sales-based milestones. We are also required to pay mid-single-digit percentage tiered royalties on future net sales of products which are developed and approved as defined by the agreements, if any. Our royalty obligations as to a particular licensed product will be payable, on a country-by-country and product-by-product basis, until the later of (a) the date of expiration of the last to expire valid claim within the licensed patents that covers the manufacture, use or sale, offer to sell, or import of such licensed product by us or a sublicense in such country, (b) loss of regulatory exclusivity, or (c) 10 years after the first commercial sale of the applicable licensed product in the applicable country. These agreements expire at the end of all royalty obligations to Amgen and, upon expiration, the licenses will be fully paid, royalty-free, irrevocable and non-exclusive. QIMR Berghofer Agreements – In October 2015, we entered into an exclusive license agreement and a research and development collaboration agreement with QIMR Berghofer Medical Research Institute (“QIMR Berghofer”). Under the terms of the license agreement, we obtained an exclusive, worldwide license to develop and commercialize allogeneic cytotoxic T-lymphocyte (“CTL”) therapy programs utilizing technology and know-how developed by QIMR Berghofer. In consideration for the exclusive license, we paid $3.0 million in cash to QIMR Berghofer, which was recorded as research and development expense in our statement of operations and comprehensive loss. Under the research and development collaboration agreement, we are required to reimburse the cost of agreed upon development activities. These payments are expensed on a straight-line basis over the term of the agreement and resulted in research and development expense of $0.3 million and $0.6 million for the three and six months ended June 30, 2016, respectively. The agreement also provides for various milestone and royalty payments to QIMR Berghofer based on achievement of certain developmental milestones and future product sales, if any. Milestones and royalties under each of the above agreements are contingent upon future events and will be recorded as expense when it is probable that the milestones will be achieved or royalties are due. As of June 30, 2016 and December 31, 2015, there were no outstanding obligations for milestones and royalties to MSK, Amgen and QIMR Berghofer.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2016
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies License and Collaboration Agreements Certain potential payments related to our license and collaboration agreements, including milestone and royalty payments, are detailed in Note 5. As the achievement of these milestones and royalties are currently not fixed and determinable, such commitments have not been included in our balance sheets. Other Research and Development Agreements We may enter into contracts in the normal course of business with clinical research organizations for clinical trials, with contract manufacturing clinical supplies, and with other vendors for pre-clinical studies, supplies and other services for our operating purposes. These contracts generally provide for termination on Operating Leases In December Operating Leases Periods Ending December 31, (in thousands) 2016 $ 747 2017 1,295 2018 981 2019 734 2020 614 Thereafter 208 Total operating lease commitments $ 4,579 Less income from sublease 141 Net minimum operating lease commitments $ 4,438 Rent expense for the three months ended June 30, 2016 and 2015 was $0.3 million and $0.1 million, respectively. Rent expense for the six months ended June 30, 2016 and 2015 was $0.6 million and $0.2 million, respectively. Indemnification Agreements In the normal course of business, we enter into contracts and agreements that contain a variety of representations and warranties and provide for indemnification for certain liabilities. The exposure under these agreements is unknown because it involves claims that may be made against us in the future but have not yet been made. To date, we have not paid any claims or been required to defend any action related to our indemnification obligations. However, we may record charges in the future as a result of these indemnification obligations. We also have indemnification obligations to our directors and executive officers for specified events or occurrences, subject to some limits, while they are serving at our request in such capacities. There have been no claims to date and we believe the fair value of these indemnification agreements is minimal. Accordingly, we did not record liabilities for these agreements as of June 30, 2016 and December 31, 2015. Contingencies From time to time, we may be involved in legal proceedings, as well as demands, claims and threatened litigation, which arise in the normal course of our business or otherwise. The ultimate outcome of any litigation is uncertain and unfavorable outcomes could have a negative impact on our results of operations and financial condition. Regardless of outcome, litigation can have an adverse impact on us because of the defense costs, diversion of management resources and other factors. We are not currently involved in any material legal proceedings.

Stockholders' Equity

Stockholders' Equity	6 Months Ended
	Jun. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	7. Stockholders’ Equity The following shares of common stock were reserved for future issuance as of June 30, 2016: Total Shares Reserved 2014 Equity Incentive Plan 9,247,174 2014 Employee Stock Purchase Plan 663,667 Total reserved shares of common stock 9,910,841 Restricted Stock Awards In August 2012 and March 2013, our chief executive officer and one other Atara employee purchased RSAs with certain vesting conditions. As of June 30, 2016, 1,245,240 of these shares had vested and are reported as shares outstanding in the financial statements. The remaining 90,144 shares are expected to fully vest in 2016. Stock-based compensation expense related to the RSAs is recorded using accelerated graded vesting model and was $0.1 million and $0.3 million for the three months ended June 30, 2016 and 2015, respectively, and $0.2 million and $0.6 million for the six months ended June 30, 2016 and 2015, respectively. The unrecognized stock-based compensation expense related to unvested RSAs was $34,000 as of June 30, 2016 and this expense is expected to be recognized in 2016. The aggregate intrinsic value of unvested RSAs was $2.0 million as of June 30, 2016. 2014 Equity Incentive Plan (2014 EIP) Our 2014 EIP permits the issuance of options, RSAs, RSUs and other types of awards to employees, directors and consultants. In June 2016, our stockholders approved an increase of 4,000,000 shares to the shares reserved for issuance under the 2014 EIP. As of June 30, 2016, a total of 9,247,174 shares of common stock were reserved for issuance under the 2014 Plan, of which 4,370,250 were subject to outstanding options and RSUs and 4,876,924 shares were available for future grant. Restricted Stock Awards and Units The following is a summary of RSA and RSU activity under our 2014 EIP: RSAs RSUs Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2015 48,317 $ 0.40 427,605 $ 7.86 Granted — 642,697 $ 15.78 Forfeited — (44,333 ) $ 11.83 Vested (32,211 ) $ 0.40 (103,400 ) $ 6.31 Unvested as of June 30, 2016 16,106 $ 0.40 922,569 $ 13.36 Vested and unreleased 23,582 Outstanding as of June 30, 2016 946,151 As of June 30, 2016, there was $8.9 million of unrecognized stock-based compensation expense related to RSUs that is expected to be recognized over a weighted average period of 1.7 years. The aggregate intrinsic value of the RSUs outstanding as of June 30, 2016 was $21.3 million. Under our RSU net settlement procedures, we withhold shares at settlement to cover the minimum payroll withholding tax obligations. During the six months ended June 30, 2016, we settled 104,653 RSUs, of which 101,355 RSUs were net settled by withholding 3,298 shares. The value of the RSUs withheld was approximately $53,000 based on the closing price of our common stock on the settlement date. These amounts were remitted to the appropriate taxing authorities and have been reflected as a financing activity in our condensed consolidated statements of cash flows. Stock Options The following is a summary of option activity under our 2014 EIP: Number of shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2015 3,137,529 $ 25.81 Granted 462,600 $ 19.99 Exercised (2,444 ) $ 11.82 Forfeited or expired (173,586 ) $ 25.39 Outstanding as of June 30, 2016 3,424,099 $ 25.12 6.04 $ 7,120 Vested and expected to vest as of June 30, 2016 3,424,099 $ 25.12 6.04 $ 7,120 Exercisable as of June 30, 2016 702,649 $ 22.36 5.60 $ 2,451 Aggregate intrinsic value represents the difference between the closing stock price of our common stock on June 30, 2016 and the exercise price of outstanding, in-the-money options. As of June 30, 2016, there was $36.5 million of unrecognized stock-based compensation expense related to stock options that is expected to be recognized over a weighted average period of 3.0 years. Options for 2,444 and 2,595 shares of our common stock were exercised during the six months ended June 30, 2016 and 2015, with an intrinsic value of $26,000 for each period. The fair value of each option issued was estimated at the date of grant using the Black-Scholes valuation model. The following table summarizes the weighted-average assumptions used as inputs to the Black-Scholes model, and resulting weighted-average grant date fair values of stock options granted during the period indicated: Six months ended June 30, 2016 Six months ended June 30, 2015 Employees Consultants Employees Consultants Assumptions: Expected term (years) 4.5 6.9 4.5 6.9 Expected volatility 68.9 % 66.1 % 71.2 % 70.1 % Risk-free interest rate 1.4 % 1.7 % 1.3 % 1.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 11.04 $ 11.57 $ 15.31 $ 16.66 Options granted 453,600 9,000 800,099 2,000 Total estimated grant date fair value $ 5,006,000 $ 104,000 $ 12,248,000 $ 33,000 The estimated fair value of stock options that vested during the six months ended June 30, 2016 and 2015 was $6.3 million and $1.1 million, respectively. 2014 Employee Stock Purchase Plan (“2014 ESPP”) As of June 30, 2016, there were 663,667 shares authorized for issuance under the 2014 ESPP. The 2014 ESPP permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering periods. Eligible employees can purchase shares of the Company’s common stock at 85% of the lower of the fair market value of the common stock at (i) the Stock-based Compensation Expense Total stock-based compensation expense related to all employee and non-employee awards was as follows: Three months ended June 30, Six months ended June 30, 2016 2015 2016 2015 (in thousands) (in thousands) Research and development $ 2,434 $ 1,251 $ 4,681 $ 2,540 General and administrative 2,701 1,318 5,179 2,513 Total stock-based compensation expense $ 5,135 $ 2,569 $ 9,860 $ 5,053

Summary of Significant Accoun13

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2016
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (U.S. GAAP) and following the requirements of the Securities and Exchange Commission (SEC) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2015, and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s consolidated financial information. The results of operations for the six month period ended June 30, 2016 are not necessarily indicative of the results to be expected for the full year or any other future period. The balance sheet as of December 31, 2015 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete consolidated financial statements.
Significant Risks and Uncertainties	Significant Risks and Uncertainties We have incurred significant operating losses since inception and have relied on public and private equity financings to fund our operations. As of June 30, 2016, we had an accumulated deficit of $133.6 million. As we continue to incur losses, our transition to profitability will depend on the successful development, approval and commercialization of product candidates and on the achievement of sufficient revenues to support our cost structure. We may never achieve profitability, and unless and until we do, we will need to continue to raise additional capital. Management expects that our cash, cash equivalents and short-term investments as of June 30, 2016 will be sufficient to fund our planned operations through 2018.
Concentration of Credit Risk and Other Uncertainties	Concentration of Credit Risk and Other Uncertainties We place cash and cash equivalents in the custody of financial institutions that management believes are of high credit quality, the amount of which at times, may be in excess of the amount insured by the Federal Deposit Insurance Corporation. We also have short-term investments in money market funds, U.S. Treasury, government agency and corporate debt obligations, commercial paper and asset-backed securities, which can be subject to certain credit risk. However, we mitigate the risks by investing in high-grade instruments, limiting our exposure to any one issuer, and monitoring the ongoing creditworthiness of the financial institutions and issuers. We are subject to certain risks and uncertainties and believe that changes in any of the following areas could have a material adverse effect on future financial position or results of operations: our ability to obtain future financing; regulatory approval and market acceptance of, and reimbursement for, our product candidates, if approved; performance of third-party clinical research organizations and manufacturers upon which we rely; development of sales channels; protection of our intellectual property; litigation or claims against us based on intellectual property, patent, product, regulatory or other factors; and our ability to attract and retain employees necessary to support our growth.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Significant estimates relied upon in preparing these financial statements include estimates related to clinical trial accruals and stock-based compensation expense. Actual results could differ materially from those estimates.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In January 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Loss (Topic 326): Measurement of Credit Losses on Financial Instruments

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	6 Months Ended
	Jun. 30, 2016
Earnings Per Share [Abstract]
Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share	The following table represents the potential common shares issuable pursuant to outstanding securities as of the related period end dates that were excluded from the computation of diluted net loss per common share as their inclusion would have an antidilutive effect: As of June 30, 2016 2015 Unvested RSAs 90,144 416,207 Unvested RSUs 922,569 564,821 Vested and unvested options 3,424,099 470,094 ESPP share purchase rights 4,048 —

Financial Instruments (Tables)

Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2016
Financial Instruments Disclosure [Abstract]
Estimated Fair Value and Related Valuation Input Hierarchy of Financial Assets Comprised Solely of Available-for-Sale Securities Measured on Recurring Basis	Our financial assets are measured at fair value on a recurring basis using the following hierarchy to prioritize valuation inputs, Level 1: Quoted prices in active markets for identical assets or liabilities that we have the ability to access Level 2: Observable market based inputs or unobservable inputs that are corroborated by market data such as quoted prices, interest rates and yield curves Level 3: Inputs that are unobservable data points that are not corroborated by market data We review the fair value hierarchy classification on a quarterly basis. Changes in the ability to observe valuation inputs may result in a reclassification of levels of certain securities within the fair value hierarchy. We recognize transfers into and out of levels within the fair value hierarchy in the period in which the actual event or change in circumstances that caused the transfer occurs. There have been no transfers between Level 1, Level 2, and Level 3 in any periods presented. The following tables summarize the estimated fair value and related valuation input hierarchy of our financial assets measured on a recurring basis, which were comprised solely of available-for-sale securities as of each period end: Total Total Total Total Amortized Unrealized Unrealized Estimated As of June 30, 2016: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 21,377 $ — $ — $ 21,377 U.S. Treasury obligations Level 2 81,460 114 (3 ) 81,571 Government agency obligations Level 2 29,716 25 (6 ) 29,735 Corporate debt obligations Level 2 154,173 109 (47 ) 154,235 Asset-backed securities Level 2 7,595 4 (2 ) 7,597 Total available-for-sale securities 294,321 252 (58 ) 294,515 Less amounts classified as cash equivalents (26,314 ) — — (26,314 ) Amounts classified as short-term securities $ 268,007 $ 252 $ (58 ) $ 268,201 Total Total Total Total Amortized Unrealized Unrealized Estimated As of December 31, 2015: Input Level Cost Gain Loss Fair Value (in thousands) Money market funds Level 1 $ 16,364 $ — $ — $ 16,364 U.S. Treasury obligations Level 2 599 — (1 ) 598 Government agency obligations Level 2 36,480 1 (88 ) 36,393 Corporate debt obligations Level 2 203,767 8 (339 ) 203,436 Commercial paper Level 2 999 — — 999 Asset-backed securities Level 2 61,304 2 (102 ) 61,204 Total available-for-sale securities 319,513 11 (530 ) 318,994 Less amounts classified as cash equivalents (22,259 ) — 1 (22,258 ) Amounts classified as short-term securities $ 297,254 $ 11 $ (529 ) $ 296,736
Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity	The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows: As of June 30, 2016 As of December 31, 2015 Amortized Estimated Amortized Estimated Cost Fair Value Cost Fair Value (in thousands) (in thousands) Maturing within one year $ 213,377 $ 213,431 $ 211,311 $ 211,059 Maturing in one to five years 80,944 81,084 108,202 107,935 Total available-for-sale securities $ 294,321 $ 294,515 $ 319,513 $ 318,994

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2016
Commitments And Contingencies Disclosure [Abstract]
Future Operating Lease Commitments	future minimum commitments for our operating leases were as follows: Operating Leases Periods Ending December 31, (in thousands) 2016 $ 747 2017 1,295 2018 981 2019 734 2020 614 Thereafter 208 Total operating lease commitments $ 4,579 Less income from sublease 141 Net minimum operating lease commitments $ 4,438

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	6 Months Ended
	Jun. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Common Stock Reserved for Future Issuance	The following shares of common stock were reserved for future issuance as of June 30, 2016: Total Shares Reserved 2014 Equity Incentive Plan 9,247,174 2014 Employee Stock Purchase Plan 663,667 Total reserved shares of common stock 9,910,841
Summary of RSU Activity and Restricted Stock Award	The following is a summary of RSA and RSU activity under our 2014 EIP: RSAs RSUs Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Unvested as of December 31, 2015 48,317 $ 0.40 427,605 $ 7.86 Granted — 642,697 $ 15.78 Forfeited — (44,333 ) $ 11.83 Vested (32,211 ) $ 0.40 (103,400 ) $ 6.31 Unvested as of June 30, 2016 16,106 $ 0.40 922,569 $ 13.36 Vested and unreleased 23,582 Outstanding as of June 30, 2016 946,151
Summary of Stock Option Activity	The following is a summary of option activity under our 2014 EIP: Number of shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2015 3,137,529 $ 25.81 Granted 462,600 $ 19.99 Exercised (2,444 ) $ 11.82 Forfeited or expired (173,586 ) $ 25.39 Outstanding as of June 30, 2016 3,424,099 $ 25.12 6.04 $ 7,120 Vested and expected to vest as of June 30, 2016 3,424,099 $ 25.12 6.04 $ 7,120 Exercisable as of June 30, 2016 702,649 $ 22.36 5.60 $ 2,451
Summary of Options Estimated with Weighted Average	The fair value of each option issued was estimated at the date of grant using the Black-Scholes valuation model. The following table summarizes the weighted-average assumptions Six months ended June 30, 2016 Six months ended June 30, 2015 Employees Consultants Employees Consultants Assumptions: Expected term (years) 4.5 6.9 4.5 6.9 Expected volatility 68.9 % 66.1 % 71.2 % 70.1 % Risk-free interest rate 1.4 % 1.7 % 1.3 % 1.6 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % Fair Value: Weighted-average estimated grant date fair value per share $ 11.04 $ 11.57 $ 15.31 $ 16.66 Options granted 453,600 9,000 800,099 2,000 Total estimated grant date fair value $ 5,006,000 $ 104,000 $ 12,248,000 $ 33,000
Schedule of Stock-based Compensation, Related to Employee and Nonemployee Awards	Total stock-based compensation expense related to all employee and non-employee awards was as follows: Three months ended June 30, Six months ended June 30, 2016 2015 2016 2015 (in thousands) (in thousands) Research and development $ 2,434 $ 1,251 $ 4,681 $ 2,540 General and administrative 2,701 1,318 5,179 2,513 Total stock-based compensation expense $ 5,135 $ 2,569 $ 9,860 $ 5,053

Description of Business (Detail

Description of Business (Detail)	6 Months Ended
	Jun. 30, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporation state	Delaware
Entity incorporation date	Aug. 1, 2012

Summary of Significant Accoun19

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Accumulated deficit	$ 133,551	$ 98,110

Net Loss per Common Share - Ant

Net Loss per Common Share - Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share (Detail) - shares	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Unvested RSAs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	90,144	416,207
Unvested RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	922,569	564,821
Vested and Unvested Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	3,424,099	470,094
ESPP Share Purchase Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	4,048

Financial Instruments - Estimat

Financial Instruments - Estimated Fair Value and Related Valuation Input Hierarchy of Financial Assets Comprised Solely of Available-for-Sale Securities Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	$ 294,321	$ 319,513
Total Unrealized Gain	252	11
Total Unrealized Loss	(58)	(530)
Total Fair Value	294,515	318,994
Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	21,377	16,364
Total Fair Value	21,377	16,364
U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	81,460	599
Total Unrealized Gain	114
Total Unrealized Loss	(3)	(1)
Total Fair Value	81,571	598
Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	29,716	36,480
Total Unrealized Gain	25	1
Total Unrealized Loss	(6)	(88)
Total Fair Value	29,735	36,393
Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	154,173	203,767
Total Unrealized Gain	109	8
Total Unrealized Loss	(47)	(339)
Total Fair Value	154,235	203,436
Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost		999
Total Fair Value		999
Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	7,595	61,304
Total Unrealized Gain	4	2
Total Unrealized Loss	(2)	(102)
Total Fair Value	7,597	61,204
Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	26,314	22,259
Total Unrealized Loss		(1)
Total Fair Value	26,314	22,258
Amounts Classified As Short-Term Securities
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	268,007	297,254
Total Unrealized Gain	252	11
Total Unrealized Loss	(58)	(529)
Total Fair Value	$ 268,201	$ 296,736

Financial Instruments - Amortiz

Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Amortized cost
Maturing within one year, Amortized cost	$ 213,377	$ 211,311
Maturing in one to five years, Amortized cost	80,944	108,202
Total available-for-sale securities, Amortized cost	294,321	319,513
Estimated Fair value
Maturing within one year, Estimated fair value	213,431	211,059
Maturing in one to five years, Estimated fair value	81,084	107,935
Total available-for-sale securities, Estimated fair value	$ 294,515	$ 318,994

License and Collaboration Agr23

License and Collaboration Agreements - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Oct. 31, 2015	Sep. 30, 2014	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Related Party Transaction [Line Items]
Payments under exclusive option license agreement		$ 1,250,000
Share issued under the agreements		59,761
Issuance of common stock related to technology licensing option		$ 750,000
Research and development expense		$ 2,000,000	$ 12,991,000	$ 11,507,000	$ 24,238,000	$ 17,274,000
Payments under license agreement					4,500,000
Potential milestone payments payable			33,000,000		33,000,000
Contractual obligations due MSK, Amgen and QIMR			0		0		$ 0
QIMR Berghofer Medical Research Institute
Related Party Transaction [Line Items]
Research and development expense			300,000		600,000
Payments under license agreement	$ 3,000,000
Maximum
Related Party Transaction [Line Items]
Potential milestone payments upon achievement of milestone			58,000,000		58,000,000
Potential milestone payments related to sales			104,000,000		104,000,000
License Agreement With Amgen
Related Party Transaction [Line Items]
Payments under license agreement			$ 200,000	$ 300,000	$ 400,000	$ 700,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Dec. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Loss Contingencies [Line Items]
Sublease to third party date				May 31, 2016
Rental expense		$ 300,000	$ 100,000	$ 600,000	$ 200,000
Liabilities related to indemnification agreements	$ 0	$ 0		$ 0
South San Francisco California Lease
Loss Contingencies [Line Items]
Lease expiration date				Apr. 30, 2021
Letter of credit expiration date				Dec. 31, 2016
South San Francisco California Lease | Restricted Cash
Loss Contingencies [Line Items]
Letter of credit issued	$ 200,000
South San Francisco Sublease
Loss Contingencies [Line Items]
Lease expiration date				Jan. 31, 2017
Westlake Village California Lease
Loss Contingencies [Line Items]
Lease expiration date				Apr. 30, 2019

Commitments and Contingencies25

Commitments and Contingencies - Schedule of Future Minimum Commitments for Operating Leases (Detail) $ in Thousands	Jun. 30, 2016USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,016	$ 747
2,017	1,295
2,018	981
2,019	734
2,020	614
Thereafter	208
Total operating lease commitments	4,579
Less income from sublease	141
Net minimum operating lease commitments	$ 4,438

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Future Issuance (Detail)	Jun. 30, 2016shares
Class Of Stock [Line Items]
Total reserved shares of common stock	9,910,841
2014 Equity Incentive Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	9,247,174
2014 Employee Stock Purchase Plan
Class Of Stock [Line Items]
Total reserved shares of common stock	663,667

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 5,135,000	$ 2,569,000	$ 9,860,000	$ 5,053,000
shares of common stock ,reserved for issuance	9,910,841		9,910,841
Aggregate intrinsic value	$ 7,120,000		$ 7,120,000
Common stock, shares exercised			2,444	2,595
Option intrinsic value, exercised			$ 26,000	$ 26,000
Estimated fair value of stock option, vested			$ 6,300,000	1,100,000
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional Common Stock, Capital Shares Reserved for Future Issuance	4,000,000		4,000,000
shares of common stock ,reserved for issuance	9,247,174		9,247,174
Outstanding options and RSUs	4,370,250		4,370,250
Aggregate number of awards available for grant to be issued	4,876,924		4,876,924
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 100,000
shares of common stock ,reserved for issuance	663,667		663,667
Aggregate number of awards available for grant to be issued	663,667		663,667
Percentage of employees purchase price of common stock			85.00%
Employee stock purchase plan description			(i) the beginning of a 12-month offering period, or (ii) at the end of one of the two related 6-month purchase periods
Employee stock purchase plan offering commenced start date	Jun. 1, 2016
Employee stock purchase plan offering commenced end date	May 31, 2017
Shares purchased	0
2014 Employee Stock Purchase Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock may be issued or transferred, value			$ 25,000
Unvested RSAs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested restricted stock awards	16,106		16,106		48,317
Unrecognized stock-based compensation expense	$ 34,000		$ 34,000
Aggregate intrinsic value	$ 2,000,000		$ 2,000,000
Unvested RSAs | Prior to Recapitalization
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vested restricted stock awards			1,245,240
Unvested restricted stock awards	90,144		90,144
Expected vesting date			Dec. 31, 2016
Unvested RSAs | Service And Performance Conditions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 100,000	$ 300,000	$ 200,000	$ 600,000
Unvested RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested restricted stock awards	922,569		922,569		427,605
Unrecognized stock-based compensation expense	$ 8,900,000		$ 8,900,000
Aggregate intrinsic value	21,300,000		$ 21,300,000
Unrecognized stock-based compensation weighted average recognition period			1 year 8 months 12 days
Restricted stock units, settled			104,653
Restricted stock units, issued net of tax withholdings			101,355
Restricted stock units withheld for tax obligations			3,298
Restricted stock units withheld for tax obligations, value			$ 53,000
Vested and Unvested Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation weighted average recognition period			3 years
Unrecognized stock-based compensation	$ 36,500,000		$ 36,500,000

Stockholders' Equity - Summary

Stockholders' Equity - Summary of RSU Activity and Restricted Stock Award (Detail)	6 Months Ended
	Jun. 30, 2016$ / sharesshares
Unvested RSAs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested as of December 31, 2015	48,317
Vested	(32,211)
Unvested as of June 30, 2016	16,106
Unvested as of December 31, 2015 | $ / shares	$ 0.40
Vested | $ / shares	0.40
Unvested as of June 30, 2016 | $ / shares	$ 0.40
Unvested RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested as of December 31, 2015	427,605
Granted	642,697
Forfeited	(44,333)
Vested	(103,400)
Unvested as of June 30, 2016	922,569
Vested and unreleased	23,582
Outstanding as of June 30, 2016	946,151
Unvested as of December 31, 2015 | $ / shares	$ 7.86
Granted | $ / shares	15.78
Forfeited | $ / shares	11.83
Vested | $ / shares	6.31
Unvested as of June 30, 2016 | $ / shares	$ 13.36

Stockholders' Equity - Summar29

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Outstanding, Number of shares, beginning balance	3,137,529
Granted, Number of shares	462,600
Exercised, Number of share	(2,444)	(2,595)
Forfeited or expired, Number of share	(173,586)
Outstanding, Number of shares, ending balance	3,424,099
Vested and expected to vest, Number of shares	3,424,099
Exercisable, Number of shares	702,649
Outstanding, Weighted Average Exercise Price, beginning balance	$ 25.81
Granted, Weighted Average Exercise Price	19.99
Exercised, Weighted Average Exercise Price	11.82
Forfeited or expired, Weighted Average Exercise price	25.39
Outstanding, Weighted Average Exercise Price, ending balance	25.12
Stock options vested and expected to vest, Weighted Average Exercise Price	25.12
Exercisable, Weighted Average Exercise Price	$ 22.36
Outstanding, Weighted Average Remaining Contractual Term	6 years 15 days
Stock options vested and expected to vest, Weighted Average Remaining Contractual Term	6 years 15 days
Exercisable, Weighted Average Remaining Contractual Term	5 years 7 months 6 days
Aggregate intrinsic value	$ 7,120
Vested and expected to vest, Aggregate Intrinsic Value	7,120
Exercisable, Aggregate Intrinsic Value	$ 2,451

Stockholders' Equity - Summar30

Stockholders' Equity - Summary of Estimated Weighted-Average Assumptions (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted	462,600
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	4 years 6 months	4 years 6 months
Expected volatility	68.90%	71.20%
Risk-free interest rate	1.40%	1.30%
Expected dividend yield	0.00%	0.00%
Weighted-average estimated grant date fair value per share	$ 11.04	$ 15.31
Options granted	453,600	800,099
Total estimated grant date fair value	$ 5,006,000	$ 12,248,000
Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 10 months 24 days	6 years 10 months 24 days
Expected volatility	66.10%	70.10%
Risk-free interest rate	1.70%	1.60%
Expected dividend yield	0.00%	0.00%
Weighted-average estimated grant date fair value per share	$ 11.57	$ 16.66
Options granted	9,000	2,000
Total estimated grant date fair value	$ 104,000	$ 33,000

Stockholders' Equity - Schedu31

Stockholders' Equity - Schedule of Stock-based Compensation Related to All Employee And Non-employee Awards (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 5,135	$ 2,569	$ 9,860	$ 5,053
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,434	1,251	4,681	2,540
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,701	$ 1,318	$ 5,179	$ 2,513